1. Field of the Invention
This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved water-flooding operations involving the injection of an oil slug containing a perferentially oil-soluble alcohol and a preferentially oil-soluble ether-linked anionic surfactant.
2. Discussion of the Prior Art
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding, which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus, it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant waterflooding or as low tension waterflooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. Thus far, many such waterflooding applications have employed anoinic surfactants. For example, a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, February 1973, pp. 205-210, describes a promising technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as water-soluble biopolymer in a graded concentration in order to provide a mixiumum viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
One problem encountered in waterflooding with certain of the anionic surfactants such as the sulfonates is the lack of stability of these surfactants in a so-called "high brine" environment. These surfactants tend to precipitate from solution in the presence of monovalent salts such as soldium chloride in concentrations in excess of about 2 to 3 weight percent and in the presence of much lower concentrations of divalent metal ions such as calcium and magnesium ions. Typically, divalent metal ion concentrations of about 50 to 100 ppm and above cause precipitation of the petroleum sulfonates.
Various surfactant formulations which contain anionic sulfonates that tolerate high salinities and/or high divalent metal concentrations have been proposed for use in high brine environments. Thus, U.S. Pat. No. 3,827,497 and U.S. Pat. No. 3,890,239 disclose a surfactant composition comprising a mixture of an organic sulfonate and a sulfated or sulfonated oxyalkylated alcohol and a polyalkylene glycol alkyl ether. The sulfonate is exemplified by the formula EQU C.sub.12 -C.sub.15 O(CH.sub.2 CH.sub.2 O).sub.m SO.sub.3 ---Na.sup.+,
where m is an average of 3.
Another surfactant waterflooding process employing an ether-linked anionic surfactant in high salinity environments is disclosed in U.S. Pat. No. 3,977,471. This patent discloses the use of an R.sub.1 hydrocarbyl ether-linked R.sub.2 hydrocarbyl sulfonate. The R.sub.1 lipophilic base is provided by a benzene, toluene, or xylene radical having an alkyl substituent containing 6-24 carbon atoms and the R.sub.2 group linking the sulfonate group with the alkoxy ether group is a C.sub.1 -C.sub.8 alkyl, cycloalkyl, alkene or aryl radical. The R.sub.2 hydrocarbyl group may be substituted with a hydroxy group or a C.sub.1 -C.sub.8 aliphatic group. Dodecyl dimethylbenzene ether propane sulfonate is specifically disclosed by patentees. The patent process is said to be particularly useful in reservoirs having high salinity brines, i.e. salinities of 2% or more, and the dodecyl dimethylbenzene ether propane sulfonate is said to be stable in saline solutions containing from 7-14 weight percent sodium chloride. U.S. Pat. No. 4,161,983 discloses the use of similar sulfonates. In addition to the use of the ether-linked surfactants in aqueous surfactant solutions, U.S. Pat. NO. 3,977,471 discloses that the surfactants may be used in liquid hydrocarbon solvents or in microemulsions. The microemulsion contains a refined or crude oil, an aqueous medium, and the previously described ether-linked surfactant. In addition, the microemulsion may contain a co-surfactant. Particularly effective co-surfactants are said to include alcohols, ethoxylated alcohols, sulfated ethoxylated alcohols, sulfonated ethoxylated alcohols, ethoxylated phenols, sulfated ethoxylated phenols and synthetic sulfonates. Alcohols disclosed in the patent for use as microemulsion co-surfactants include C.sub. 3 -C.sub.20 aliphatic alcohols such as isopropanol, isobutanol, tertiary butanol, amyl alcohols, hexanols, octanols, and dodecanols. The patentees specifically disclose flooding with microemulsion systems containing 91% to 93% water containing 6% sodium chloride, 5% oil, 1% to 3% dodecyl dimethylbenzene ether propane sulfonate, and 1% butyl alcohol.
Yet another surfactant waterflooding process involving the use of an anionic ether-linked surfactant is disclosed in U.S. Pat. No. 4,018,278. The ether-linked surfactants employed include sulfonated polyethoxylated aliphatic alcohols and sulfonated polyethoxylated alkylphenols. The patentee discloses the use of the ether-linked sulfonates alone or as a co-surfactant with anionic surfactants such as petroleum sulfonates and in high brine environments, e.g., in a solution having a salinity of 183,000 ppm and a total hardness of 9400 ppm. The surfactant system is said to be particularly useful at temperatures in excess of 120.degree. F. Also, U.S. Pat. No. 4,217,957 discloses an aqueous mixture of an alkylbenzene ethoxylated propylene sulfonate and the dialkylbenzene derivative, useful in waterflooding.